This invention relates to optical devices, more particularly to a fiber-optic dual frequency shifter and a fiber-optic interferometer.
This invention also relates to a method of generating optical interference signals in which reliable sensor signals are obtained independent of polarization states of the optical waves in optical fibers.
Fiber-optic interferometric sensors obtain sensor signals by interfering two light waves that passed through an interferometer on different optical paths. In general, two light waves that passed through the fiber-optic interferometer on different optical paths experience different polarization change because of the birefringence which is intrinsic or induced by external influences such as fiber-bendings. One of the problems involved in the operation of fiber-optic interferometric sensors is therefore that the intensity of the interference signal is non-uniform and varies in time. For example, the intensity of the interference signal is extremely low for signal processing or completely disappears when the polarization states of the interfering optical waves are mutually orthogonal.
To solve this problem, prior arts employ the following methods of controlling the polarization state.
Method Using a Squeezer
If a side of an optical fiber is squeezed, refractive indices in the direction of squeezing pressure and in its normal direction will change to have different values. Adjusting fiber birefringence by this squeezing method can make the intensity of interference signals high enough because it induces appropriate polarization change in the light waves passing through the fiber.
FIG. 1 is a schematic cross-sectional view of a polarization controller using a squeezer. Referring to FIG. 1, two parallel plates 20 contacting the side of an optical fiber 10 squeezes the fiber 10. The pressure applied to the fiber 10 forms a fast axis and a slow axis therein since refractive indices in the direction of squeezing pressure and in its normal direction become different. The squeezer 5 comprises the two parallel plates 20 and means for pressuring (not shown). A series of three squeezers, the squeezing directions of which are twisted by 45 degrees between adjacent ones, can produce optimum polarization state by adjusting the respective squeezing pressures.
This method can be used in a study of small laboratory level. However, it is not adequate for automatic application, and causes the complexity of system configuration when applied to sensor arrays using only one common optical detector since a series of three squeezers should be equipped with one sensor.
Method Using a Loop-type Polarization Controller
The loop-type polarization controller uses the birefringence induced in an optical fiber when the fiber is bent in a loop shape. Radial direction of the loop and direction normal to the loop plane become birefringence axes. Appropriately adjusting the radius of a loop can make the loop a quarter-wave plate.
FIG. 2 schematically shows the configuration of a loop-type polarization controller 25.
Referring to FIG. 2, two loops are arranged in series and each loop can be rotated in T-direction along the axis of the linear fiber portion. If the angle between two loop planes is optimally adjusted, maximum interference can be realized.
This method provides more convenient way of controlling polarization compared to the method using a squeezer. However, it is also inadequate for automatic and sensor array applications.
Method Using Input Polarization Scanning
When optical waves passed through two different fiber optical paths interfere with each other, the interference visibility depends upon the polarization state of input light as well as the birefringence of the fiber. In this method, three different polarization states, all of which can not cause the interference signals to disappear simultaneously are input one by one with time in pulse mode, and then the output signals are separated by a detector to produce a maximum signal. In principle, the three polarization states satisfying this condition can be represented by three points on the Poincare Sphere, where the lines from the center of the Sphere to the three points are mutually perpendicular.
This method is adequate for automatic and sensor array applications, however, has the complexity of signal processing since one maximum signal should be selected from the comparison of three signals.
There are other methods using such as polarization masking or polarization switching besides the above-described methods. However, these methods also have problems of complex signal processing and noise generation.
The object of the present invention is to introduce a novel device called fiberoptic dual frequency shifter and to provide two complementary interferometric signals so that they do not suffer polarization-induced signal fading simultaneously by using this device in fiber-optic interferometers. Generally, any polarization state in the fiber is a linear sum of the two orthogonal eigen polarization states of the fiber. In the case of usual fiber-optic frequency shifter, the optical frequencies of the two eigen polarization states are frequency shifted to the same amounts. But according to the present invention, the two eigen polarization components are differently frequency shifted. The dual fiberoptic frequency shifter may be made of an elliptic core optical fiber or a polarization maintaining optical fiber.
The present invention also provides a fiber-optic interferometer adapting the dual frequency shifter in one of the two optical paths. This configuration produces two beat interference signals of different frequencies whose amplitudes are always complementary. Due to polarization fluctuation, the amplitudes vary with time but they are never faded simultaneously and the larger signal is selected and used for sensing the measurand.